The present invention relates generally to reconfigurable high-impedance surfaces. More particularly, the present invention relates to reconfigurable artificial magnetic conductors.
Recent advances in communication technology have lead to the creation of surfaces that approximate perfect magnetic conductors, in which the tangential magnetic field impinging on the surface is forced to be zero. These surfaces, however, only approximate perfect magnetic conductors over a limited band of frequencies, as defined by the xc2x190xc2x0 reflection phase bandwidth, and are named artificial magnetic conductors, or AMCs.
An example of a known AMC is shown in FIG. 1. The AMC 100 illustrated in FIG. 1 is fabricated using conventional circuit technology and features an electrically-thin, planar, periodic structure, referred to as a frequency selective surface (FSS) 102. The periodic structure includes capacitive patches 110 that are connected to a conductive ground plane 106 by means of metal vias or posts 108. The posts 108 pass through a spacer layer 104 that consists of a dielectric material having a relatively low permeability. While the spacer layer 104 is typically 10-40 times thicker than the FSS 102, one advantage of AMCs is that the entire structure (FSS, spacer layer, and ground plane) has a much smaller thickness than the free space wavelengths of the frequencies over which the AMC operates, i.e. the wavelength at resonance. In addition, the periodicity of the periodic structure is much smaller than the free space wavelength, typically being {fraction (1/12)} to {fraction (1/40)} of the wavelength at resonance.
The resonant frequency of an AMC is defined to be that frequency or frequencies at which the reflection phase angle for a plane wave at normal incidence is zero degrees. For single resonant frequency AMCs as shown in FIG. 1, the resonant frequency is defined as fo=1/(2xcfx80{square root over (LC)}) where the inductance L is the product of the height of the spacer layer containing the vias times the permeability of the medium which comprises the spacer layer. For simple air filled spacer layers, the inductance may be approximated by L=xcexco where xcexco is the permeability of free space, and h is the height of the spacer layer, or the distance between the solid metal conductor of the ground plane and the lower side of the capacitive FSS. The effective sheet capacitance of the FSS is denoted as C, and is measured in Farads per unit square. The resonant frequency of an AMC may be adjusted by varying either or both the inductance and the capacitance of the AMC.
The AMC permits wire antennas to be well matched, in terms of impedance, and radiate efficiently when the antennas are placed in close proximity to the FSS, usually less than {fraction (1/100)} of the wavelength from the surface. The physical structure of the AMC yields an equivalent transmission line model shown in FIG. 2a and the equivalent lumped circuit model shown in FIG. 2b. In FIGS. 2a and 2b, the capacitive FSS is modeled as a shunt capacitance, while the spacer layer is modeled as a transmission line or inductor. These circuit models accurately represent the surface impedance seen by an incident plane wave.
These size reductions are advantageous as most wireless communications applications desire the antenna ground plane to be as small and lightweight as possible so that it may be readily integrated into physically small, lightweight platforms such as radiotelephones, personal digital assistants and other mobile or portable wireless devices. Practically, the relationship between the instantaneous bandwidth of an AMC with a non-magnetic spacer layer and its thickness is given by       BW          f      0        =      2    ⁢    π    ⁢          xe2x80x83        ⁢          h              λ        0            
where xcexo is the free space wavelength at resonance where a zero degree reflection phase is observed. Thus, to support a wide instantaneous bandwidth, the AMC thickness must be relatively large. For example, to accommodate an octave frequency range (BW/f0=0.667), the AMC thickness must be at least 0.106xcexo, corresponding to a physical thickness of 1.4 inches at a center frequency of 900 MHz. This thickness is too large for many practical applications.
Accordingly, there is a need for an artificial magnetic conductor, which allows for a wider frequency coverage for a given AMC thickness than the AMC depicted in FIG. 1. This problem has been addressed in presently pending application Ser. No. 09/845,666 filed Apr. 30, 2001, herein incorporated by reference. In that application, the resonant frequency, fo, of the AMC is electronically adjusted or tuned by controlling the effective sheet capacitance C of its FSS layer. This type of reconfigurable AMC (RAMC) uses integration of varactor or PIN diodes into a single layer FSS where the bias voltage is applied using a resistive lattice which is coplanar with the diode array to adjust the capacitance. Thus, the inter-patch capacitance between the patches is varied in this RAMC. Other RAMCs may change the capacitance of the effective circuit by translating overlapping capacitive patches on different layers and altering the overlap between the two sets of patches.
However, such RAMCs, while having a wide frequency coverage for a given AMC thickness, may have a problem with intermodulation distortion as power levels become significant. Intermodulation distortion is always present when the radio frequency (RF) electronic control devices are used to tune the capacitance in the communication systems. The solid state approaches used above produce intermodulation products in the radiated spectrum when antennas are integrated into RAMCs. It would thus be advantageous to provide an RAMC and that has a broad tuning bandwidth of at least an octave while simultaneously minimizing intermodulation distortion.
In the present RAMC, at least one of the inductance or capacitance is varied. The present RAMC has such a broad tuning bandwidth and minimization of intermodulation distortion. The use of RF electronics is reduced, which permits the device to operate in the presence of high RF fields and currents. In addition, intermodulation products in the RAMC are expected to be very low due to the absence of nonlinear devices.
In a first embodiment, the artificial magnetic conductor (AMC) comprises a ground plane and a frequency selective surface (FSS). The FSS has capacitive patches, at least some of which are electrically connected with the ground plane. The distance between the FSS and the ground plane is variable. The position of one or both of the FSS and ground plane may be adjustable. The distance between the FSS and ground plane may be limited to less than the maximum distance between the FSS and ground plane. The distance between the two may be reversibly varied, varied once and only once, or varied in a single direction. Furthermore, the distance may be varied in discrete amounts or continuously by a linear actuator such as a manually (i.e. by hand not via a motor) or with the aid of a motor.
The AMC may also include spring contact probes or spring tabs, which are used to connect the capacitive patches of the FSS with the ground plane. The spring tabs may be thin, bent in one or more positions, freely or permanently contact the FSS. Threaded shafts may be used to engage with vias in either of the FSS and ground plane to vary the distance between the two. Any movable member (either or both of the FSS and ground plane) may be reinforced by a buttressing mechanism, such as a board stiffener. The board stiffener may be non-metallic. The spacer layer between the FSS and ground plane may be filled substantially with air or a dielectric having a relatively low permittivity.
In a second embodiment, the equivalent transmission line circuit of the AMC has an inductor of variable inductance in parallel with a capacitor. The conductor may have a constant capacitance. The inductance may be defined by a permeability multiplied by a multiplier. The permeability may be constant while the multiplier is variable. The resonant frequency of the AMC may be adjustable over at least a 3:1 or about a 10-15% tuning ratio by varying the inductance. The inductance may be either continuously variable or variable by discrete amounts. Further, the inductance may be either reversibly variable, variable once and only once, or variable only in a single direction, increasing or decreasing.
In a third embodiment, the AMC contains the ground plane and two FSS layers. At least one of these is movable and at least one has a constant position. As in the above embodiments, the FSS layers have at least one set of capacitive patches associated with each layer. This is to say that one or more of the FSS layers may have multiple layers of capacitive patches disposed at different positions on that FSS (most frequently opposing surfaces).
In a fourth embodiment, a method of effecting a broad tuning bandwidth of at least an octave while simultaneously minimizing intermodulation distortion in an AMC comprises varying a distance between a ground plane and a FSS of the AMC.
In a fifth embodiment, a method of effecting a broad tuning bandwidth of at least an octave while simultaneously minimizing intermodulation distortion in an AMC an equivalent transmission line circuit comprises varying an inductance of the equivalent lumped circuit model of the AMC.
Many different devices and communication systems may use the AMCs described above, for example: an antenna, a telephone, a personal digital assistant, a portable wireless device, or a computer.